1. Field of the Invention
The present invention relates to an external resonator type wavelength-tunable light source used, for example, in the optical measurement field. This application is based on Japanese patent application No. Hei 8-94927 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
The structure of a conventional external resonator type wavelength-tunable semiconductor laser (LD) light source is shown in FIG. 5. In the figure, LD 11 as an optical amplifier has a Fabry-Perot type structure, one end face of which being coated with an antireflection film (i.e., AR coating) 11, and is driven by LD drive circuit 12. The outgoing beam from the antireflection-film side of LD 11 is transformed into a collimated beam by lens 13, and is input into diffraction grating 14. The diffraction grating 14 is used as a wavelength-selection reflector, and has a function to reflect a beam (portion) having a specified wavelength determined according to the incident angle, selected from among the incident collimated beam, in a specified direction.
That is, the diffraction grating 14 and the other end face without an antireflection film of LD 11 form a resonator; thus, it is possible to perform LD oscillation by re-inputting the selected beam (by diffraction grating 14) into LD 11.
On the other hand, The laser beam output from the side without antireflection film 111 of LD 11 is transformed into a collimated beam by lens 15; passes through optical isolator 16; is converged by lens 17; and is then input as an output beam into output fiber 18 for transmission. Here, optical isolator 16 has a function of preventing the beam reflected from the output fiber 18 side from returning to LD 11 which is an optical amplifier.
The direction of diffraction grating 14 can be adjusted for any angle with respect to the incident optical axis by using angle adjustment mechanism 19. Therefore, by performing a driving control using wavelength-tunable drive circuit 20 for the angle adjustment mechanism 19, diffraction grating 14 can be rotated to be set in any direction, and thus it is possible to arbitrarily change the selected wavelength (i.e., Bragg's wavelength) and thereby to perform wavelength variation within the gain range of LD 11.
In addition, it is possible to move diffraction grating 14 in a parallel direction with respect to the incident optical axis by using parallel motion mechanism 21. Accordingly, diffraction grating 14 can be moved in parallel with the direction of the optical axis of the resonator by performing a driving control using positioning drive circuit 22 with respect to parallel motion mechanism 21, and thus the resonance wavelength can be arbitrarily changed.
In this way, in the wavelength-tunable LD light source having the above-explained structure, it is possible to simultaneously perform continuous wavelength-variation without mode hopping by controlling the Bragg's wavelength variation according to angle adjustment and the resonance wavelength variation according to positioning adjustment with respect to diffraction grating 14.
Another structure of the external resonator type wavelength-tunable LD light source for also realizing a narrow spectral line width has been disclosed in Japanese Patent Application, First Publication, No. Hei 6-112583. In FIG. 6, parts which are identical to those shown in FIG. 5 are given identical reference numbers, and duplicate explanations will be omitted here.
In this external resonator type wavelength-tunable light source, LD 11 with an antireflection film on one end face is also used as an optical amplifier. On the other hand, as an external resonator, (i) beam splitter 23 for splitting a beam from LD 11 and diffraction grating 14 and (ii) total reflection mirror 24 for totally reflecting the split beam from the beam splitter 23 in the incident direction are used with above-explained diffraction grating 14; angle adjustment mechanism 19; and parallel motion mechanism 21, to form optical resonance reflector A. This optical resonance reflector A is used as a reflector at the antireflection-film 111 side of LD 11.
This external resonator construction by using the optical resonance reflector A has a steep reflection characteristic with respect to the wavelength; thus, the oscillation wavelength of LD 11 is subjected to optical negative feedback. Therefore, spectral line width can be narrowed. The continuous wavelength variation can also be obtained by using, in principle, the same construction of the wavelength-tunable LD light source with the diffraction grating (as shown in FIG. 5), and by performing the angle adjustment of diffraction grating 14 and the length adjustment of the resonator, simultaneously.
However, in this structure of the wavelength-tunable LD light source of the optical resonance reflector type, there exist two resonators: one is a resonator consisting of total reflection mirror 24 and diffraction grating 14, and the other is a resonator consisting of diffraction grating 14 and the end face without an antireflection film (111) of LD 11. LD 11 is inserted in one resonator; therefore, the refractive index of LD 11 affects the length of the resonator, by which the states of variations of both resonators are not the same at the time of wavelength variation.
In the structures of the conventional wavelength-tunable LD light sources as shown in FIGS. 5 and 6, LD oscillation can be realized by equipping LD 11, that is, an optical amplifier, in the resonator. However, LD 11 has a problem in which the refractive index thereof changes according to operating conditions such as current, temperature, the oscillation wavelength, and the like, and thus stability of the oscillation wavelength is inferior. In order to obtain a stable oscillation wavelength, it is necessary to consider driving current, temperature, etc., with respect to LD 11, in addition to maintaining a fixed length of the external resonator.
Furthermore, as for LD light sources, another method for performing light-intensity modulation with respect to the output beam has been proposed. In this case, if LD driving current is directly modulated, the refractive index of the LD is changed. Consequently, in addition to the intensity modulation of the output beam, the oscillation wavelength is also modulated. Therefore, when the light-intensity modulation with a stable oscillation wavelength is required, it is necessary to connect an external optical modulator to the system, instead of performing direct modulation of the LD driving current.
On the other hand, in the case of performing continuous wavelength variation without mode hopping, it is necessary to coordinate the changes of (i) Bragg's wavelength (.lambda..sub.Gr) determined by diffraction grating 14 and (ii) resonance wavelength (.lambda..sub.FP) determined according to the m-order of the longitudinal mode of the resonator, so as to obtain the same amounts of both wavelength changes. In order to satisfy this condition, the angle adjustment of diffraction grating 14 and the length adjustment of the resonator must be performed while maintaining the wavelength difference between the Bragg's wavelength and the resonance wavelength as .lambda./2 or less.
As a method for realizing this specific condition, a sine bar mechanism, which is used in spectroscopes, has been proposed. FIG. 7 shows a system arrangement using the sine bar mechanism. In FIG. 7, parts which are identical to those shown in FIG. 5 or 6 are given identical reference numbers.
Diffraction grating 14 is driven and rotated by an angle adjustment mechanism (not shown) which is adjustable with respect to the angle and thus has a function to change the Bragg's wavelength. In addition, the angle adjustment mechanism itself is disposed on the parallel motion mechanism and hence can be moved in parallel in the direction of the axis of the resonator, by which a function to change the resonance wavelength is provided.
The diffraction grating 14 is rotated via a sine bar contacted with a contact stand (detailed indication omitted in Figures). When the parallel motion mechanism is shifted in parallel according to a driving control by the positioning drive circuit, diffraction grating 14 is also automatically rotated via the sine bar.
In the structure of the sine bar, the relationship between the lengths of the resonator and the sine bar must satisfy the following equations (1)-(3). EQU .lambda..sub.Gr =2.multidot.d.multidot.sin .theta. (1) EQU .lambda..sub.FP =2.multidot.n.multidot.L/m=2.multidot.L.sub.A .multidot.sin .theta./m (2) EQU .DELTA..lambda.=.lambda..sub.Gr -.lambda..sub.FP =2.multidot.sin .theta..multidot.(d-L.sub.A /m) (3)
Here, .lambda..sub.Gr is the Bragg's wavelength selected by diffraction grating 14, d indicates the spacing between grooves of diffraction grating 14, .lambda..sub.FP is the resonance wavelength determined by the external resonator, n indicates the refractive index inside the external resonator, L indicates the length of the external resonator, m indicates the order of the resonance longitudinal mode of the external resonator, L.sub.A indicates the arm length of the sine bar, .theta. means the incident angle of the beam input into diffraction grating 14, and .DELTA..lambda. indicates the difference between the Bragg's wavelength .lambda..sub.Gr and the resonance wavelength .lambda..sub.FP.
When the parallel motion is performed while satisfying the above conditions, the resonance-wavelength variation using the sine bar structure and the Bragg's wavelength variation using diffraction grating 14 have the same amount with respect to the wavelength change; thus, it is possible to continuously vary the wavelength. Even though the sine bar mechanism is not used, such continuous wavelength variation is possible if the incident angle (on diffraction grating 14) and the length of the resonator are controlled so as to satisfy the above-explained equations. However, it is very difficult to separately and continuously control the adjustments of the incident angle and the length of the resonator over a wide range.
In addition, the refractive index of LD 11 changes according to the driving current, temperature, and the oscillation wavelength, as described above. The variations of the refractive index due to the driving current and temperature become approximately linear, while the variation of the refractive index due to the oscillation wavelength becomes non-linear. Therefore, even if the linear variations of the physical length of the resonator and the Bragg's wavelength of diffraction grating 14 are correctly performed, the actual change of the optical length of the resonator becomes non-linear. Accordingly, if the actual wavelength variation width is greater than a certain wavelength range, the difference between the resonance wavelength and the Bragg's wavelength becomes .lambda./2 or more. In this case, the wavelength variation is accompanied with mode hopping.
It is also possible to perform the continuous wavelength variation if a non-linear variation with respect to the length adjustment of the resonator, corresponding to the wavelength variation, is performed instead of performing the linear variation. However, non-linear characteristics of the refractive index for the wavelength are different according to different driving currents of LD 11; thus, it is also very difficult to perform the wavelength variation with non-linear adjustment of the length of the resonator.
In the conventional diffraction grating type wavelength-tunable LD light source as shown in FIG. 5, a variation can be proposed in which a semiconductor optical amplifier with both end faces processed so as to have antireflection coatings is disposed instead of LD 11. In this case, wavelengths of beams reflected by diffraction grating 14 have a center wavelength corresponding to the Bragg's wavelength according to multiple interference; thus, phase matching is achieved to a certain degree.
However, phase matching in the case of the multiple interference is inferior in comparison with the phase conditions in the resonance characteristics. Therefore, even though the beam reflected from diffraction grating 14 is incident on the optical amplifier and is amplified while passing through the amplifier, it is impossible to obtain a beam having a narrow spectral line width and high light intensity such as the beam obtained at the LD oscillation.
In the conventional light-source structure in which the LD as an optical amplifier is provided in the resonator, there is a problem in that the refractive index characteristics are changed according to variations of the driving current, temperature, the oscillation wavelength, etc., of the LD, and thus the effective optical length as the length of the resonator is also changed, as described above.
In particular, the variation of the refractive index with respect to the oscillation wavelength is non-linear; thus, a continuous wavelength variation over a wide band cannot be obtained even though a linear variation of the length of the resonator is performed. Additionally, when the wavelength variation is performed with a different driving current, there occurs a problem in which the characteristics of the continuous wavelength variation are changed in a manner such that mode hopping occurs at a different wavelength.
Furthermore, if the LD driving current is directly modulated for modulating the light intensity of the output light, the refractive index of the LD is changed; thus, the oscillation wavelength is also modulated in addition to the light-intensity modulation of the optical output. Therefore, if the light-intensity modulation with a stable oscillation wavelength is required, it is necessary to connect an external optical modulator instead of the direct modulation of the LD driving current. In this case, there is a problem in that the cost of the light source is increased.